In general, a direct current (DC) motor exhibits a linear relationship between applied voltage and speed. The linear relationship provides simplicity in speed control and a broad range for speed control. The DC motor includes a brush as a required element for maintaining unidirectional torque. Because of the brush, the DC motor has disadvantages such as poor suitability for high-speed operation, frequent maintenance due to abrasion of the brush, and excessive noise.
A brushless DC (BLDC) motor has been developed to overcome the disadvantages of the conventional DC motor. As opposed to the conventional DC motor, the BLDC motor comprises of a stator having a coil wound thereon and a rotator having a permanent magnet. A magnetic flux from the permanent magnet of the rotator interacts with a magnetic flux generated by a current flow in the coil of the stator. In the BLDC motor, the current flow in the coil of the stator is controlled such that the angle between the magnetic flux of the stator and the magnetic flux of the rotator is approximately 90° to provide a rotational force. Because a BLDC motor has no brush, it removes the above disadvantages of the conventional DC motor and retains advantages thereof. Thus, BLDC motors have been widely utilized in recent years.
To avoid using the brush, the BLDC motor uses inverter switching devices to maintain unidirectional torque. The inverter switching devices control where the magnetic flux of the stator is generated. To appropriately control magnetic flux, it is necessary to determine the rotator's position relative to the stator coils to properly coordinate the switching operations of inverter switching devices so that the angle between the magnetic flux of the stator and the magnetic flux of the rotator will be such that rotational force is developed. At this time, to detect the position of the rotator, a sensor such as a Hall sensor may be used to directly measure the rotator position. However, owing to environmental factors such as temperature and pressure, a sensorless approach is mainly used. The position of the rotator is inferred through measuring a back electromotive force voltage in the coils of the stator, thereby detecting a zero cross point (ZCP).
A ZCP is detected by comparing a reference voltage with a back electromotive force voltage induced in a certain phase of the stator. If a ZCP is detected, an interrupt is generated and phase commutation occurs after 30 electrical degrees from detection of the ZCP.
In a conventional 4-pole, 6-slot, surface permanent magnet, concentrated winding type BLDC motor, there are two 15-degree intervals having no change of magnetic flux. At these intervals, the back electromotive force voltage is zero, causing uneven ZCP detection. Because of the uneven ZCP detection, uneven detection of rotator position occurs and phase commutation does not conform to rotator position. The uneven detection of rotator position and irregular phase commutation generate a phase current ripple. The phase current ripple can cause vibrations or an abnormal operation of the BLDC motor. Shutdown of the BLDC motor may ultimately occur. Thus, performance of the BLDC motor may be degraded.